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AE64 TELECOMMUNICATION SWITCHING SYSTEMS JUN 2015

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Solution Marks

Q.2 a. Discuss the various functions of a switching system.

Ans: Page No.56 &57 in text Book II

b.List out the advantages and disadvantage of strowger switching system

Ans: Advantages:

i) Inexpensive for small system

ii) Highly reliable due to the distributed nature of equipment.

Disadvantages:

1. As this switching involves heavy mechanical displacements, regular maintenance

by the skilled technicians is necessary.

2. It is not feasible to select an alternate route for interoffice calls, if all the trunks are

busy as the switching is by step through various selectors.

3. Step by step switching is limited to dial pulses. For touchtone telephones, special

devices have to be introduced between line finder and first selector to convert the tones

into dial pulses.

4. If calling rate is high, heavy operation is performed by the system and the life time

of the system is less.

5. The last two digits of the called line numbers are specifically determined by their

location on the connector. Congestion could arise when the switching system is heavily

loaded.

6. The capacity of switching system reduces if codes of different numbers are allotted

to various subscribers, such as fire service, police ambulance, fault regorts, directory

enquiry, operator assistance etc. In certain cases, the exchange capacity may be

reduced from 10000 to even 6000 customer lines.

7. The strowger system can accept only 7 to 9 pulses in 1 second. Hence if we dial fast,

the system cannot give correct performance.

Q.3 a. A group of 7 trunks is offered 4E of traffic, find (a) the grade of

service (b)the probability that only one trunk is busy (c) the

probability that only one trunk is free and (d) the probability that at

least one trunk is free.

Ans: N = 7, A = 4E

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b. Define loss system and delay system.

Ans: The type of system by which a blocked call is simply refused and is lost is

called loss system. In the second type of system, a blocked call remains in the system

and waits for a free line. This type of system is known as delay system.

c. Define Cent Call Seconds (CCS) and Grade of Service (GOS).

Ans: Cent call seconds:

It is also referred as hundred call seconds. CCS as a measure of traffic intensity is valid

only in telephone circuits. CCS represents a call time product. This is used as a

measure of the amount of traffic expressed in units of 100 seconds. Sometimes call

seconds (CS) and call minutes (CM) are also used as a measure of traffic intensity. The

relation between erlang and CCS is given by

1E = 36 CCS = 3600 CS = 60 CM

Grade of Service (GOS):

For non-blocking service of an exchange, it is necessary to provide as many lines as

there are subscribers. But it is not economical. So, some calls have to be rejected and

retried when the lines are being used by other subscribers. The grade of service refers

to the proportion of unsuccessful calls relative to the total number of calls. GOS is

defined as the ratio of lost traffic to offered traffic.

GOS =Blocked Busy Hour calls/Offered Busy Hour calls

Q.4 a. A three stage switching structure supports 100 inlets and 400

outlets. Find

(i) the number of cross points (ii) the number of primary and

secondary switches used in the design.

Ans: We know that

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m = M/M+N and n = N/M+N

m = 100

1. if m=5, n=20, there are :

20 primary switches of size 5 x 5

5 secondary switches of size 20 x 20

20 tertiary switches of size 5 x 20

2. if m = 4, n =16, there are:

25 primary switches of size 4 x 4

4 secondary switches of size 25 x 25

25 tertiary switches of size 4 x 16

b. What are single stage and multistage switching networks? Compare the

strengths and weaknesses of each.

Sr.

No.

Single stage Multistage

1 Inlet to outlet connection is through a

single cross point.

Inlet to Outlet connection is

through

multiple cross points

2 Use of single cross point per connection

results in better quality link.

Use of multiple cross points

may degrade the quality of a

connection.

3 Each individual cross point can be used

for only one inlet/outlet pair connection.

Same cross point can be used

establish connection between

a number of inlet/outlet pairs.

4 A specific cross point is needed for each

specific connection.

A specific connection may be

established by using sets of

cross points.

5 If a cross points fails, associated

connection cannot be establish-

There is no redundancy.

Alternative cross-points and

paths are available.

6 Cross points are inefficiently used. Only

one cross point in each row or column of a

square or triangular switch matrix is even

in use, even if all the lines are active.

Cross points are used

Efficiently

7 Number of cross points is

Prohibitive

Number of cross points

is reduced significantly

8 The network is non blocking

in character

The network is blocking

in character

Q.5 a. Draw and explain Space Division Switching in detail.

Ans: Space Switches:

Connections can be made between incoming and outgoing PCM highways by means of

a cross point matrix of the form shown in Fig. However, different channels of an

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incoming PCM frame may need to be switched by different cross points in order to

reach different destinations. The cross point is therefore a two-input AND gate. One

input is connected to the incoming PCM highway and the other to a connection store

that produce a pulse at the required instant. A group of cross points gates can be

implemented as an integrated circuit, for example by using a multiplexer chip.

Fig. shows a space switch with k incoming and m outgoing PCM highways, each

carrying n channels. The connections store for each column of cross points is a

memory with an address location for each time-slot, which stores the number of the

cross points to be operated in that time slot. This number is written into the address by

the controlling processor in order to setup the connection. The numbers are read out

cyclically, in synchronism with the incoming PCM frame. In each time slot, the

number stored at the corresponding store address is read out and decoding logic

converts this into a pulse or a single lead to operate the relevant cross point.

Since a cross point can make a different connection in each of the n time-slots, it is

equivalent to n cross points in a space division network. The complete space switch is

thus equivalent to n separate k x m switches in a space division switching network.

b. Explain and compare T-S-T and S-T-S switching techniques.

Ans: STS Switching.

In STS switching, the time stage is sandwiched between two space arrays. The digital

switching system ITS 4/5 of USA (1976) uses the STS switching configuration. It

handles 3000 trunks and accommodates 1500 Erlangs of traffic. Fig. shows the space

time- space (S-T-S) switching network for M incoming and outgoing PCM highways.

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Establishing a path through an STS switch requires finding a time switch array with an

available unit’s access during the incoming time slot and an available read access

during the desired outgoing time slot. The input side space stage as well as the output

side space stage is free to utilise any free time switch modules. In the diagram shown in

Fig the time slot 2 is connected to the TSM 2 where the time slot allotted is 16 and

passed to the (M – 1)th line of output space array. Thus the path is provided. This

structure is of non-blocking nature.

TST Switching:

In TST switching the space stage is sandwiched between two time stage switches. Of

all the multistage switching, TST is a popular one. The principle of operation of TST

switching is shown in Fig. In figure, two flows of time slots, one for each direction are

connected together.

The information arriving at the incoming link of TDM channel is delayed in the· inlet

times stage until an appropriate path through the space stage is available. Then the

information is transferred through the space stage to the appropriate outlet time stage.

Here the information is held until the desired outgoing time slot occurs. Any space

stage time slot can be used to establish a connection. The space stage operates in a time

divided fashion, independently of the external TDM links. There are many alternative

paths between a prescribed input and output unlike a two stage network which has only

one fixed path.

Q.6 a. Define Stored Program Control (SPC) and explain the distributed

SPC in detail with the help of a diagram.

Ans: In stored program control systems, a program or set of instructions to the

computer is stored in its memory and the instructions are executed automatically one

by one by the processor. Carrying out the exchange control functions through programs

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stored in the memory of a computer led to this name.

Distributed SPC:

The introduction of distributed SPC enabled customers to be provided with a wider

range of services than those available with centralised and electromechanical switching

system. Instead of all processing being performed by a one or two processor in

centralised switching, functions are delegated to separate small processors (referred as

regional processors). But central processors are still required to direct the regional

processors and to perform more complex tasks. The distributed SPC offers better

availability and reliability than the centralised SPC. Entire exchange control functions

may be decomposed either horizontally or vertically for distributed processing. In

vertical decomposition, the exchange environment is divided into several blocks and

each block is assigned to a processor that performs all control functions related to that

block of equipments. In horizontal decomposition, each processor performs one or

some of the exchange control functions. Figure shows the distributed control where

switching equipment is divided into parts, each of which has its own processor.

b. Enlist & explain the various steps involved in processing a call.

Ans:

1. Idle state. At this state, the subscriber handset is in ‘on-hook’ condition. The

exchange is ready to detect the call request from the subscriber.

2. Call request identification. The exchange identifies a line requiring for a service.

When the handset is lifted, current flows in the line called seize signal indicates the call

request.

3. Providing dial tone. Once the seize signal is received, an exchange sends a dial tone

to the calling subscriber to dial the numbers.

4. Address analysis. Once the first digit received, the exchange removes the dial tone

and collects all numbers. Then the address is analysed for the validity of the number,

local, STD or ISD etc. If the number is invalid, a recorded message may be sent to the

calling subscriber and terminates call request.

5. Called line identification. The exchange determines the required outgoing line

termination from the address that it has received.

6. Status of called subscriber. The called line may be busy or free or unavailable or

even out of service. In the case of PBX, where the customer have a group of lines, the

exchange tests each termination until either it finds a free one or all one found busy.

For busy, number unobtainable or the handset off hook, a status signal or call progress

signal is sent to the calling subscribers for line termination. Now the exchange resumes

idle state.

7. Ringing. Once, the exchange finds the called subscriber is free, power ringing is

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provided to the called subscriber and audible ringing to the calling subscriber.

8. Path setup. When the called subscriber lifts his handset, the line is looped and

ringing is removed. Once the conversation started, the exchange completes the

connections between the subscribers.

9. Supervision. The exchange supervises the connection to detect the end of the call

for charging.

10. Clear signal. Once the need for connection is over, either customer may replace his

handset. It causes the line current seize and provides a clear signal to exchange. If the

calling subscriber replaces his phone set, the clear signal sent to the exchange is called

clear forward signal. If called subscriber do first, the clear signal is called clear

backward signal.

Q.7 a. Describe the architecture of SS7 common channel signalling

network with the help of a neat labelled diagram.

Ans: A block schematic diagram of the CCITT no. 7 signalling system is shown in

fig. Signal messages are passed from the central processor of the sending exchange to

the CCS system. This consists of the microprocessor based subsystem. The signalling

control subsystems, the signalling termination subsystem and the error control

subsystem. The signalling control subsystem structures the messages in the appropriate

format and queues them for transmission. When there are no messages to send, it

generates filler messages to keep the link active. Messages then passed to the signalling

termination sub system, where complete signal units (SU) are assembled using

sequence numbers and check bits generated by the error control subsystem. At the

receiving terminal, the reverse sequence is carried out. The levels are as follows:

Level 1: The Physical Layer

Level 2: The Data Link Level

Level 3: The signalling network level

Level 4: The User Part

The relationship between these levels and the layers of the OSI model is shown in Fig.

The user part encompasses layers 4 to 7 of the OSI model. Level 1 is the means of

sending bit streams over a physical path. It uses times lot 16 of a 2 Mbit/s PCM system

or times slot 24 of a 1.5 M bit/s system. Level 2 performs the functions of error control,

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link initialization, error rate monitoring, flow control and delineation of messages.

Level 3 provides the functions required for a signalling network. Each node in the

network has a single point odd, which is a 14 bit address. Every message contains a

point code of the originating and terminating nodes for that messages. Levels 1 to 3

form the message transfer part (MTP) of CCITT no. 7. Level 4 is the user part. This

consists of the processes for handling the service being supported by the signalling

system. The message transfer part is capable of supporting many different user parts.

So far, three have been defined: the telephone user part(TUE), the data user part (DUP)

and the (ISDN) user part (ISDN-UP).

b. Explain architecture of T1 link and DS-1 frame format in relation to

the PCM signalling

Ans:

In PCM systems, signalling and speech are sampled, coded and transmitted within the

frame of PCM channels. Thus, with PCM, a convenient way of transmission is

possible. The signalling information and speech information carried in the same time

slot is referred as inslot signalling. The signalling information carried in a separate time

slot is referred as outslot signalling. The timeslot of the inslot signalling is fixed at

eight bits. As one bit is used for signalling, the speech bit rate is reduced to 56 kbps

from 64 kbps and the bandwidth is reduced. Telephone channels are combined by time

division multiplexing to form an assembly of 24 or 30 channels. This is known as

primary multiplex group.

DS1 24 channel system.

DS0 is a 64-kbps signal that makes up the basis for the DS1. Twenty four DS0s

combined to produce DS1. Fig shows the architecture of a T1 link and DS-1 frame

format.

Incoming analog signals were time division multiplexed and digitized for transmission.

Each individual TDM channel is assigned 8 bits per time slot. Each frame is made of

24 × 8 bits = 192 bits plus one additional bit added to each frame to identity the fame

boundaries. Thus each frame contains 193 bits. The frame interval is 125 micro sec.

Hence the basic T1 line rate is 1.544 Mbps. This line rate has been established as the

fundamental standard for digital transmission.

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Q.8 a. Explain and compare bus and ring topology used in LAN

technology.

Ans: Bus Topology:

This topology shares a single link or path way among all users. This common single

path way is known as bus. In this topology, the link serves as a high way for all data

signals, and users connect on to the bus at their node location. In bus configurations,

network control is not centralized to a particular node. Here control is distributed

among all nodes connected to the LAN. Data transmission on a bus network is usually

in the form of small packets containing user addresses and data. When one node/user

desires to transmit data to another station, it monitors the bus to determine if it is

currently being used. If no other nodes/users are communicating over the network, the

monitoring node/user can start to transmit its data. Each node must monitor all

transmission on the network and determine which are intended for them.

Ring Topology:

In ring topology, all user nodes are connected with the physical path acting as links of a

chain and the last user node is connected back to the first node. A signal going on to

the next node must be processed by the first node, which then passes it through to the

next node. Adding a new user requires breaking the ring temporarily, inserting the new

node and then re-establishing the complete ring path.

b. Explain the advantages and services offered by Asynchronous

Transfer Mode (ATM)

Ans:

The most important advantages or benefits of the ATM are:

1. A much wider array of information can be transmitted using ATM technology, for

example, voice, data, images, CATV scans, MRI images and video conferencing.

2. ATM delivers bandwidth on demand, is not dependent on applications and works at

a data rate from 1.5 Mbps to 2 Gbps.

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3. All types of networking, from LANs to WANs and from backbone to desktop can be

integrated by ATM technology.

4. The service is connection oriented, with data transferred over a virtual circuit.

5. ATM switches are statistical multiplexing.

6. Higher quality of service.

7. Wider array of information can be handled.

8. Accepts variety of transmission media such as optical fiber or twisted pair cable.

9. Works with current LAN and WAN technologies and supports current protocols

such as TCP/IP.

ATM Services.

ATM forum specifies five types of services. They are:

1. Constant bit rate (CBR). This is used for emulating circuit switching. The cell rate

is constant with time. Telephone traffic, videoconferencing and television are the

examples that use CBR.

2. Variable bit rate–non real time (VBR–NRT). This service allows users to send

traffic at a rate that varies with time depending on the availability of user information.

Multimedia email is an example of VBR–NRT.

3. Variable bit rate–real time (VBR–RT). This service is similar to VBR–NRT, but it

is designed for applications that are sensitive to cell delay variation. Examples for real

time VBR are voice with speech activity detection (SAD) and interactive compressed

video.

4. Available bit rate (ABR). This service provides rate based flow control and is

aimed at data traffic such as file transfer and e-mail.

5. Unspecified bit rate (UBR). This class is widely used today for TCP/IP.

Q.9 a. Explain Primary Rate Interface (PRI) and Basic Rate Interface

(BRI) of ISDN.

Ans:

Basic Rate Interface (BRI):

BRI is made up of two B-channels (Bearer channels) and one D channel. Therefore the

total rate is 2B + D. B channels are 64 kbps and can be used for voice and data

communications. The D channel is 16 kbps and is used for call initialization and

signalling connections. Fig. 12.2 shows ISDN BRI. BRI is the most appropriate type of

ISDN service

for computer connections and individual use. Of three digital channels (2B + D), of

each of the channels can be used simultaneously. Thus a subscriber can perform several

communications tasks at the same time. BRI is designed to carry the most data possible

to the home through existing copper phone lines.

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Primary Rate Interface (PRI):

PRI in North America has 23 B channels and one 64 K D channel or the total rate is 23

B + 1D, having a total bandwidth 1.544 Mbps. (including 8 kbps of overhead) PRI in

rest of the world uses 30 B channels and one D channel

or 30 B + D with total rate of 2.048 Mbps. The number of B channels is limited by the

size of the standard trunk line used in the region. Fig shows the ISDN PRI interface.

Unlike BRI, PRI does not support a bus configuration and only one device can be

connected to a PRI line. A PBX, however can reallocate ISDN PRI resources on to

multiple BRI buses. Fig shows the ISDN PRI concept. A single PRI connection is

usually much less expensive than obtaining the equivalent number of B channels

through multiple BRI connections. The 1.544 Mbps of a PRI can be divided up in

many ways to meet the requirements of many users. This indicates combination of 64

kbps B channels or all capacity of B channels (In LAN to LAN case) can be used.

b. Write short notes on the following:

(i) B-ISDN

(ii) Alternative routing

Ans: B-ISDN

BISDN Configuration: Fig. shows how access to the BISDN network is

accomplished. Each peripheral device is interfaced to the access node of a BISDN

network through a broadband distant terminal (BDT). The BDT is responsible for

electrical to optical conversion, multiplexing of peripherals, and maintenance of the

subscriber’s local system. Excess nodes concentrate several BDT’s into high speed

optical fiber line directed through a feeder point into a service node. Most of the

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control function for system excess is managed by the service node, such as call

processing, administrative function and switching and maintenance functions. The

functional modules are interconnected in a star configuration and include switching,

administrative, gateway, and maintenance modules. The interconnection of the function

module is shown in Fig. The central control hub acts as the end user interface for

control signalling and data traffic maintenance. In essence, it oversees the operation of

the modules.

Subscriber terminal near the control office may by pass the excess nodes entirely and

the directly connected to the BISDN network through a service node. BISDN nodes

that used optical fiber cables can utilize much wider band width and consequently,

have higher transmission rates and offer more channel handling capacity than ISDN

systems.

ii) Alternative Routing:

Based on the assumption that the routing is made only by direct routing or tandom

routing, it is found that to route a stream of traffic, tandom route is more economical.

In fact, even greater economics are often possible if just a proportion of the traffic is

routed directly. This approach is known as alternative routing. In alternative routing,

connections should use the direct trunks (referred as high usage route), because direct

route provides better transmission quality and use fewer network facilities. If all the

direct trunks are busy, calls are routed via a tandom exchanges or alternate routes to

maintain suitably low blocking probabilities. Thus, the networks are designed to

allocate a limited number of heavily utilized trunks in the direct route and provide

alternate routes for over flow.

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TEXT BOOK

Telecommunications Switching, Traffic and Networks, J.E.Flood, Pearson Education- 2006

Telecommunication Switching Systems and Networks, Thiagarajan Viswanathan, Prentice Hall of India Pvt. Ltd, 2007

Digital Telephony, John C Bellamy, John Wiley (International Student Edition)

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